1. FIELD OF THE INVENTION
The present invention broadly relates to the transformation of RF energy field polarization in waveguide structures and, more particularly, is concerned with a dielectric slab polarizer having a novel configuration which, when the polarizer is installed in a waveguide section, provides improved impedance matching of the polarizer to the empty space of the waveguide section whereby improved transformation of RF field polarization is achieved over a wider frequency bandwidth than heretofore attained.
2. DESCRIPTION OF THE PRIOR ART
Transformation of an RF energy field from linear to circular polarization or vice versa through use of some form of a quarter-wave dielectric slab polarizer has been described in literature. See for exmaple, an article entitled "Broad-Band Quarter-Wave Plates" by Wesley P. Ayres, appearing at pages 258-261 of the October 1957 issue of the IRE Transactions on Microwave Theory and Technique Journal. Also, U.S. Pat. No. 3,858,512 which issued Oct. 28, 1958, to Edward F. Barnett, describes the use of a pair of quarter-wave dielectric slabs with a half-wave dielectric slab in an assembly for providing transformation of RF field polarization.
The theoretical broad frequency bandwidth characteristics of thick dielectric slab polarizers are well known, as recognized in the Ayres article. However, the problem of impedance matching of the thick slab to the empty space within a square or round waveguide section has prevented full realization of the theoretical bandwidth. Impedance mismatching causes unequal reflections of the two orthogonal RF field components of an electric field from the ends of the slab which increases the axial ratio to unacceptable levels and thereby limits the operating bandwidth.
This problem of reflection due to mismatching is discussed by Ayres and, as he suggests, conventional practice in slab polarizer construction is to build a quarter-wave plate having a thickness at its midsection of only about one-half of that corresponding to the optimum slab thickness to waveguide diameter ratio for the theoretical bandwidth so that the opposite ends of the plate may be provided with gradually tapered configurations which minimize the reflection problem. If a plate of optimum midsection thickness and tapered ends were utilized, the ends would have steeply tapered configurations. As found by Ayres, steep tapers, for example at an angle of almost forty-five degrees to the incident field, produce so much reflection that the slab would be useless as a quarter-wave plate.
Also, in the conventional slab construction having a midsection thickness of one-half the optimum, it has been found that most or all of the required ninety degree differential phase shift of the field components occurs within the gradual tapered end sections of the slab. Therefore, it is not feasible merely to increase the length of the tapered ends in order to combine the gradual tapered configuration of the ends with a slab midsection of optimum thickness since this would produce too much differential phase shift.